1) The present study revealed that, in the Xenopus spinal cord, the E-face of the excitatory S-type synapse was characterized by the aggregates of large-sized intramembrane particles (IMPs), 12-15 nm in diameter, 2600/um^2 in density; the complementary P-face lacked comparable IMP aggregates. In the inhibitory F-type synapse, similar aggregates of IMPs counted 3100/um^2(10-12 nm) on the P-face in contrast to 1500/um^2(8-9 nm) on the E-face. By the deep-etching study, these characteristic IMPs were found to be transmembranous structures. Taken together, these IMPs in the S- and F-type synapses may represent the receptor structure for glutamate, GABA or glycine, respectively.2) In the nicotinic synapse of the chick ciliary ganglion, the characteristic aggregates of IMPs appeared on the P-face (3300/um^2,5-12 nm). Similar features were observed in nicotinic bullfrog and guinea pig sympathetic ganglia.3) In the chick adrenal chromaffin cells, the muscarinic postsynaptic P-face contained IMPs (3770/um^2), and complementary E-face showed numerous pits and scattered IMPs. This is the first report suggesting that the muscarinic receptors are aggregated at the postsynaptic active zones, at least in the chick adrenal chromaffin cells.4) Electrical synapse or synaptic gap junciton (GJ) was found on the chick ciliary ganglion cells. These GJs were often internalized, particularly in the two to three day old chick ganglion cells. Such the internalizing GJs were not evident in the adult chick, or in the bullfrog and guinea pig adrenal chromaffin cells. Actin filaments seemed to play a pivotal role in the internalization of GJ membranes. The present histochemical and immunological studies suggest that the internalized GJ membranes and connexin, channel protein, will be degraded by lysosomal enzymes.